1. Field of the Invention
The present invention relates to a rotating electric machine system.
2. Discussion of the Background
Rotating electric machine apparatus, such as an electric generator for electromagnetically generating electric power by relative rotation of a permanent magnet and an armature, or an electric motor for generating relative rotation between a permanent magnet and an armature by interaction of the permanent magnet and a magnetic field generated by electric current supplied to the armature, are excellent in energy efficiency and have been widely used routinely with development of permanent magnets. However, in both electric motors and electric generators, optimum power is not always obtained in a wide rotational speed range because the amount of magnetic flux supplied from the permanent magnet is constant.
In the case of the electric motor, it will become difficult to control the electric motor in a high-speed rotational region because the back electromotive force (power generation voltage) becomes too high, and therefore, various methods for weakening the field strength as field-weakening control have been proposed. On the other hand, in the case of the electric generator, a constant-voltage electric generator by means of field current control only or a constant-voltage circuit in which the power generation voltage is made to be constant by a semiconductor has been exclusively used so that the power generation voltage is made to be at a predetermined level in a wide rotational-speed range.
In the case of electric motor, field-weakening control by means of phase leading electric current control has been widely adopted, but the energy efficiency and the control range are limited. Attempts have been made to perform the field control in the rotating electric machines by means of a mechanical deviation (For example, U.S. Pat. No. 7,567,006) without sacrificing the high energy efficiency upon the rotating electric machines with the permanent magnet. As the field condition can be maintained in the form of mechanical deviation, a rotating electrical machine can be actualized with high energy efficiency with the energy loss due to field excitation being kept minimum.
The other method for controlling strength of magnetic field to keep the energy loss at minimum is to change a magnetization state of the permanent magnet irreversibly, and technical propositions are made in United States Patent Applications No. 20090236923, No. 20090261774, and No. 20100171385. These proposals constitute a rotor magnetic pole facing an armature in a permanent magnet with high coercivity and a permanent magnet with low coercivity, and are going to change magnetization of the permanent magnet with low coercivity by magnetic field that an armature coil makes. However, the composition where low coercivity magnets are placed close to the armature coil has always possibility that the magnetization of the above-mentioned permanent magnet may be changed due to unforeseeable accidents, and serious concern remains in stability of the system. Moreover, the magnetization change due to high voltage and high current pulse could seriously damage electronic circuits.